Typically, an image sensor array includes a focal plane array of pixels, each one of the pixels including a photo-conversion device such as, e.g., a photogate, photoconductor, or a photodiode. FIG. 1A illustrates a cross section of a portion of a conventional CMOS imager pixel 100 having a pinned photodiode 114 as its photo-conversion device. FIG. 1B illustrates the entire pixel 100 circuit in schematic form. The photodiode 114 is adjacent to an isolation region 110, which is depicted as a shallow trench isolation (STI) region. The photodiode 114 includes an n-type region 115 underlying a p+ surface layer 116.
The photodiode 114 converts photons to charge carriers, e.g., electrons, which are transferred to a floating diffusion region 140 by a transfer transistor 119. In addition, the illustrated pixel 100 typically includes a reset transistor 121, connected to a source/drain region 136, for resetting the floating diffusion region 140 to a predetermined level (shown as Vaapix) prior to charge transference. In operation, a source follower transistor 142 (FIG. 1B) outputs a voltage representing the charge on the floating diffusion region 140 to a column line 150 (FIG. 1B) when a row select transistor 152 (FIG. 1B) for the row containing the pixel 100 is activated.
CMOS image sensor circuits, processing steps thereof, and detailed descriptions of the functions of various CMOS elements of an image sensor circuit are described, for example, in U.S. Pat. No. 6,140,630, U.S. Pat. No. 6,376,868, U.S. Pat. No. 6,310,366, U.S. Pat. No. 6,326,652, U.S. Pat. No. 6,204,524, and U.S. Pat. No. 6,333,205, assigned to Micron Technology, Inc. The disclosures of each of the forgoing patents are herein incorporated by reference in their entirety.
In the conventional pixel 100, when incident light strikes the surface of the photodiode 114, charge carriers (electrons), are generated in the depletion region of the p-n junction (between region 115 and region 116) of the photodiode 114.
As can be seen from FIGS. 1A and 1B, CMOS sensors typically use several transistors in every pixel for various functions including amplification. Although FIGS. 1A and 1B describe a four transistor (4T) design, pixel circuits are also known which have fewer, (e.g., 3T), as well as more (e.g., 5T, 6T, etc.) transistors. As pixel size is scaled down in high-resolution sensors, the area taken by these transistors and corresponding interconnects becomes significant and reduces the pixel area available for the photo-conversion device. Photo-conversion device area should be made as large as possible to increase imager sensitivity and quantum efficiency.
In addition, there are several imaging applications that require high quantum efficiency, small pixels, and have unique wavelength requirements, such as shorter visible wavelengths and ultraviolet light. Increasing the size of the photo-conversion device area is particularly important in such cases.